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Automotive RPM converter for tachometer

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Sketch_hs

New Member
I'm doing an engine conversion on a 1988 Mazda Rx7

I'm putting in a I6 engine where the oringinal motor is a rotary engine that has a tach signal like a 4 cylinder.

So how can I make the stock tach work properly with the new 6 cylinder output?

Any ideas on circuits or maybe you guys know of products already made that do this?
 

Noggin

Member
I designed a board that does this for a 4 cyl to 6 cyl swap. It was pretty easy and was microcontroller based. All you really need to do is measure the incoming frequency then bump it up 50% and spit it back out to an N-FET with a resistor pullup to 12v. At least thats what I did.
 

picbits

Well-Known Member
A bit like mine then



Although mine has a wide range of variable ratios and has been designed to be fairly bombproof. Its undergoing a redesign at the moment as the one in the picture was an early prototype but so many people wanted them that I never got round to making a "neat" version.
 

crutschow

Well-Known Member
Most Helpful Member
I designed a board that does this for a 4 cyl to 6 cyl swap. It was pretty easy and was microcontroller based. All you really need to do is measure the incoming frequency then bump it up 50% and spit it back out to an N-FET with a resistor pullup to 12v. At least thats what I did.
Do you mean 6 cyl to 4 cyl swap? I would expect a 6 cyl engine to have a higher pulse frequency output (from the distributor I assume) than a 4 cyl, so I would think you want to reduce the signal frequency, not increase it.
 

crutschow

Well-Known Member
Most Helpful Member
If you want to build you own you could use a pulse swallower circuit generated by a divide by 3 flip-flop circuit and an AND gate as shown in the attached (CD CMOS ICs shown). The divide by 3 circuit is used to inhibit (swallow) every third pulse through the AND gate giving you the 2/3 (4/6) division you need.

Of course you will need to filter the power, and provide proper buffer circuits at the chip input and output to match the input signal and provide the desired output signal.
Divide by 2-3.gif
 
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Noggin

Member
Do you mean 6 cyl to 4 cyl swap? I would expect a 6 cyl engine to have a higher pulse frequency output (from the distributor I assume) than a 4 cyl, so I would think you want to reduce the signal frequency, not increase it.
Yeah, you're right. I bumped up the period, not the frequency :) Little bit of difference between the two.

This was the (bery blurry) board I made...
 
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MikeMl

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jjjeffcoat

New Member
I have the same problem. 4cyl to a 6cyl swap (vw).

could i get someone to send me a parts list and diagram? i would like to try my hand at micro-controllers. If i need software, please also, i would take suggestions. neat tinkering huh..
instructions would be great also.
[email protected]

I designed a board that does this for a 4 cyl to 6 cyl swap. It was pretty easy and was microcontroller based. All you really need to do is measure the incoming frequency then bump it up 50% and spit it back out to an N-FET with a resistor pullup to 12v. At least thats what I did.
 
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